Microlithography is commonly used for the production of microstructured components, such as, for example, integrated circuits or LCDs. The microlithography process is often carried out in what is referred to as a projection exposure apparatus having an illumination system and a projection objective. The image of a mask illuminated by the illumination system (reticle) is in case projected by the projection objective onto a substrate (e.g., a silicon wafer) which is coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection objective to transfer the mask structure onto the light-sensitive coating on the substrate.
It is known for linearly polarised light from a laser source to be depolarised by a Hanle depolariser and a light mixing system arranged downstream of the Hanle depolariser. Such a Hanle depolariser can include at least a first wedge plate of birefringent material transparent to light of the working wavelength and typically also a second wedge plate which compensates for the beam deflection of the first wedge plate and which is made from birefringent or non-birefringent material which is transparent to light of the working wavelength. The first wedge plate of the Hanle depolariser is usually arranged in such a way that the angle between the optical crystal axis of the birefringent material and the vibration direction of the electrical field strength vector of the linearly polarised light from the laser source is substantially 45°.